subway

My friend spent 15 minutes to teaching me how to order a sandwich in the subway  at Subway. Because of my poor listening, I always could only
understand what the attendant said if he spoke quickly slowly, so I always pointed to the pictures to select food. But now, I have studied how to say different sandwiches, vegetables and sauces. I will try again next time.

I fully understood what you wrote. After the verb 'spend' we use the -ing form of a verb, not the to + infinitive. 

Subway is a proper noun, so it has a capital S.

I also changed your adverb, as in my experience it's easier to understand someone when they are talking slowly, not quickly, but that's just me!

Good job, as I said, the errors did not result in any misunderstanding.

Subway

My friend spent 15 fifteen minutes to teaching me how to order a sandwich in the at Subway. Because of my poor listening skills, I always never could understand what the attendant said if he or she spoke quickly, and so I always pointed to the pictures to make my selections food. But now, I have studied how to say pronounce the names of different sandwiches, vegetables and sauces. I will try again next time.

subway

My friend spent 15 minutes trying to teach me how to order a sandwich at Subway. Because of my poor listening skills, I could only 
understand what the attendant was saying if he spoke quickly, so I could only point to the pictures of each food item to select them. But now, I have studied how to say different sandwiches, vegetables and sauces. I will try again next time.

Great job! I understood everything that you were trying to say :) It seems like the only major mistake you made is using "always" instead of "only." It's always if you do this every time at Subway but is not appropriate here because you are only talking about one time. Still, it didn't hurt my understanding of what you meant. Keep up the good work!